Pressure sensing probe

ABSTRACT

A pressure sensing probe having an elongated housing with an elastic diaphragm located at one end of the housing. The outer face of the diaphragm is substantially flush with the outer wall surface of the housing. An interior cavity is formed in the housing with at least a portion of the inner face of the diaphragm being in communication with the interior cavity. Also in communication with the interior cavity are a fluid supply port containing a flow-restricting supply orifice, a fluid output pressure port, and a fluid exhaust port containing a flowrestricting exhaust nozzle, each of which are formed in the walls of said housing. The diaphragm is normally biased inwardly by a biasing spring having its one end attached to an armature secured to the inner face of the diaphragm. The other end of the biasing spring is attached to an armature stem guide fixedly secured within the cavity of the housing which maintains the spring in constant tension. The armature also has a stem that passes downwardly within the biasing spring and through the armature stem guide. The lower end of the armature stem acts as a flapper which when the armature stem is raised and lowered varies the flow of fluid through the exhaust nozzle positioned beneath the flapper to control the amount of fluid passing outwardly through the fluid exhaust port. A fluid supply regulator is connected to the fluid supply port and a fluid exhaust regulator is connected to the fluid exhaust port in order to control the pressure of the fluid supplied through the orifice to the fluid cavity of the probe and to control the pressure of the fluid exhausted through the nozzle from the fluid cavity of the probe. The fluid output pressure port is also connected directly to each of the pressure regulators, thereby providing a pilot pressure for each of the regulators. The function of the probe is to reproduce exactly in fluid pressure the changes in the fluid pressure to which the sensing diaphragm is exposed. The probe output pressure reading will always be greater than the actual fluid pressure if the diaphragm is preset with an inward bias by the biasing spring. This probe biasing allows negative fluid pressures to be read out on the instrument as positive fluid pressures, thus enabling it to deal with negative heads of pressure.

United States Patent [1 1 F razel [111 3,821,897 [451 July 2,1974

[ PRESSURE SENSING PROBE Wilbur H. Frazel, Riverside, R1.

[73] Assignee: General Signal Corporation, New

York, NY.

22 Filed: Oct. 17, 1972 21 Appl. No.: 298,408

[75] Inventor:

[52] US. Cl. 73/213, 73/388 BN, 137/85 Primary Examiner-Richard C.Queisser Assistant ExaminerJohn P. Beauchamp Attorney, Agent, orFirm-Barlow & Barlow 5 7 ABSTRACT A pressure sensing probe having anelongated housing with an elastic diaphragm located at one end of thehousing. The outer face of the diaphragm is substantially flush with theouter wall surface of the housing. An interior cavity is formed in thehousing with at least a portion of the inner face of the diaphragm beingin communication with the interior cavity. Also in communication withthe interior cavity are a fluid supply port containing aflow-restricting supply ori- OUTPUT gRESSURE FLUID SUPPLY 6O PRESSUREFLUID EXHAUST fice, a fluid output pressure port, and a fluid exhaustport containing a flow-restricting exhaust nozzle, each of which areformed in the walls of said housing. The diaphragm ,is normally biasedinwardly by a biasing spring having its one end attached to an armaturesecured to the inner face of the diaphragm. The other end of the biasingspring is attached to an armature stem guide fixedly secured within thecavity of the housing which maintains the spring in constant tension,The armature also has a stem that passes downwardly within the biasingspring and through the armature stem guide. The lower end of thearmature stem acts as a flapper which when the armature stem is raisedand lowered varies the 'flow of fluid through the exhaust nozzlepositioned beneath the flapper to control the amount of fluid passingoutwardly through the fluid exhaust'port. A fluid supply regulator isconnected to the fluid supply port and a fluid exhaust regulator isconnected to the fluid exhaust port in order to control the pressure ofthe fluid supplied through the orifice to the fluid cavity of the probeand to control the pressure of the fluid exhausted through the nozzlefrom the fluid cavity'of the probe. The fluid output pressure port isalso connected directly to each,

of the pressure regulators, thereby providing a pilot pressure for eachof the regulators. The function of theprobe is to reproduce exactly influid pressure the changes in the fluid pressure to which the sensingdiaphragm is exposed. The probe output pressure reading will always begreater than the actual fluid pressure if the diaphragm is preset withan inward bias by the biasing spring. This probe biasing allows negativefluid pressures to be read out on the instrument as positive fluidpressures, thus enabling it to deal with negative heads of pressure.

8 Claims, 5 Drawing Figures mnmnm 2:914 1821.897

' SHEET 1 0F 3 PILOT PRESSURE OUTPUT gRESSURE FLUID SUPPLY 60 FLUIDPRESSURE EXHAUST FlG.l

BACKGROUND OF THE INVENTION In the measurement of fluid flow in apipeline it is I common practice to install a differential producer suchas a Venturi tube which allows inferential determination of flow rate bymeasurement of pressure differential. The general configuration of theVenturi tube and other similar devices, and their theory of operationare well known in the art. Presently to measure pressures developed inthe differential producer, piezometer holes are usually drilled in thewall of the device communicating with the interior at two prescribedpoints, namely the inlet and thethroat. Pipes commonly lead from thesepressure .taps to an external secondary device which may display thedifferential pressure and/or convert it to an analogous signal which maybe pneumatic, electric, etc., in nature.

When the pipeline fluid contains settleable solid material, as in thecases of sewage, sludge, slurries, etc., trouble is often experiencedbecause the open pressure taps and pipes connecting to them eventuallybecome fouled with accumulations of solid material to the extent thataccuracy of measurement of the pressure is affected. Many means ofdealing with this difficulty are available, but most only slow but donot entirely eliminate accumulation. Others, which are capable ofpreventing accumulation in taps and piping, require constantsurveillance and maintenance to assure proper operation and may inthemselves variably bias the detected pressure and hence affectaccuracy.

SUMMARY OF THE INVENTION The pressure sensing probe is a force balancedevice which operates to maintain exact balance between the forcecreated by the fluid pressure acting on the outside of the sensingdiaphragm plus the inward force of the biasing spring, and the force ofthe fluid pressure within the housing acting on the inside of thesensing diaphragm. Adjustment is made so that with the sensing diaphragmexactly in its neutral position, the flapper is in its operatingposition close to the nozzle such that the fluid flow through the nozzleis controlled. Fluid pressure within the housing will be a function ofpressure of fluid supplied to the orifice and pressure loss across theorifice determined by orifice size andrate of flow of fluid through it.With the flapper close to the nozzle, highly restricting flow, rate offlow through the orifice will be low, loss across the orificewilltherefore be small, and housing pressure will be high, approaching fluidsupply pressure. With the flapper farther away from the nozzle,restricting flow less, rate of v flow through the orifice will behigher, loss across the orifice will therefore be greater, and housingpressure will be lower, retreating from fluid supply pressure. Supplyand exhaust pressures also have a major effect on housing pressure forany given flapper-nozzle separation.

In a condition of balance the amount of fluid allowed to flow throughthe orifice by the flapper-nozzle couple is just sufficient to create ahousing pressure to exactly balance the sensed pressure plus the biasingspring equivalent pressure. If sensed pressure increases, the systemunbalances and the flapper-nozzle separation decreases slightly torestrict the flow of the fluid,

thereby raising housing pressure enough to again achieve balance. Therequired new housing fluid pressure is produced by a slight decrease inflapper-nozzle separation. In other words, the flapper assumes a newslightly-changed position relative to the nozzle.

The inlet probe senses line pressure and the throat probe senses linepressure less differential. The quantity of real interest is thedifference between the two sensed pressures. The accuracy obtained withthese novel pressure sensing probes is of a magnitude better than havepreviously been demanded of a pressure sensing device. Only because thedesign of the probe stays within a single physical domain, pressure,with a single interfacing diaphragm, is the device able to perform theconversion so successfully. One-to-one tracking is automatic in thiscase, since a single diaphragm presents the same effective area to thefluid on the outside and the fluid on the inside. Thus no calibration isrequired. Sensitivity and accuracy of pressure balance are not dependenton magnitude of the sensed pressure.

In order for the probe, as so far described, to follow an increasedpressure applied to the outside of the diaphragm it is necessary thatthe flapper assume a new position slightly closer to the nozzle. Thisincrement of position change, no matter how small, associated with therate of the biasing spring, spring rate of the diaphragm due to itselasticity, effective area change of the diaphragm, and changes inreaction force due to fluid flow around the-repositioned flapper,results in pressure changes within the probe substantially, but notexactly, equal to pressure changes outside. For highly precisereproduction of pressure changes, this error is not tolerable,especially in cases where the measured pressure varies widely. Withgiven supply and exhaust pressures, given orifice and nozzle sizes, anda given position of the'flapper with respect to the nozzle within itsthrottling range, the pressure within the probe will be at someintermediate value between supply and exhaust pressures. Then if bothsupply and exhaust pres-.

sures are changed by exactly the same given amount, the intermediatepressure will also change by exactly the given amount without requiringany change whatsoever in flapper position relative to the nozzle, sincepressure drop across the system and therefore fluid flow through thesystem and loss across the orifice, will all remain constant. Bychanging supply and exhaust pressure in one-to-one relationship to probeoutput pressure changes, the latter can be made to follow pressurechanges acting on the outside surface of the diaphragm without anysustained position changes of the flapper and the associated errors.

The output pressure of the probe is used as a pilot pressure for thepair of fluid control regulators, one

spring-biased to maintain its downstream pressure (up stream pressure ofthe orifice) at a value always a selected increment above its pilotpressure. Similarly the exhaust regulator is biased to maintain itsupstream pressure (downstream pressure of the nozzle) at a ,value alwaysa selected increment below its pilot pressure. Both regulators controlfluid flow to accomplish this regulation. I

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view illustrating apair of,pressure sensing probes having their diaphragms inserted intoholes provided respectively at the inlet and throat areas of a Venturitube; i

FIG. 2 is a partial cross section view illustrating how the pressuresensing probe is secured to the hole in the wall of the Venturi tube;

FIG. 3 is a cross sectional view illustrating the pressure sensingprobe;

FIG. 3A is a magnified view of the end-of the pressure sensing probehaving the diaphragm located therein;

FIG. 4 is a cross section view of an alternative embodiment of apressure sensing probe.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of thedrawings, a pair of pressure sensing probes generally designated areschematically illustrated as being secured to the outer wall of aVenturi tube 20. One probe could be attached to the inlet area of theVenturi tube and the other probe attached to the throat area. One mannerin which the individual probes can be attached to the Venturi tube isbest illustrated in FIG. 2. In that figure it is shown that a boss 21would be formed in the outer face of wall 22 of the Venturi tube in thearea where the pressure sensing probe is to be attached. The boss wouldhave'a hole drilled down from its top all the way through to theinterior surface 23 of the Venturi tube and this hole would have anouter bore 24 of a greater diameter than the inner bore 26. The diameterof the inner bore would be substantially the same as the outer diameterof the neck 12 of the pressure sensing probe. A portion of outer bore 24would be threaded as at 25 to receive the threading on gland bushing 28.Bushing 28 would have a bore 29 whose diameter would be substantiallythe same as a portion of the pressure sensing probe that would beinserted through bore 29. An annular channel 30 would be formed in thisbore to receive an O-ring 32 in order to provide a pressure tight sealto prevent Another probe would be similarly installed at the sec-.

ond prescribed pressure sensing point in the Venturi tube. With thesehighly accurate pressure transfer devices at the two appropriate pointsin the Venturi tube, a secondary device connected to these probes willreceive exact pressures and there will be no possibility of fouling ofsecondary devices and/or the connecting pipesby pipeline fluidj Onepossible construction of the pressure sensing probe is illustrated inFIGS. 3 and 3A and they will serve as basis for an explanation of theoperation of the probe. The probe 10 has a sealed housing 13 formed withan interior cavity 14 within which the movable components of the probeare located. A source of suitable fluid under appropriate pressureisconnected to a fluid supply port in the housing. Fluid pressure entersthe probe through orifice 15a and passes into the interiorcavity 14 ofthe probe. This cavity is also bounded by the output pressure port 16,the interior walls 17 of the probe, diaphragm l8 and fluid exhaust port19. Diaphragm 18 is suitably attached to an armature 40 into which isscrewed an armature stem 41. The

diaphragm, its armature and the armature stem move as a unit in responseto differential pressures on the two sides of the diaphragm. Thearmature stem is of such length that with the diaphragm initsneutralposition (so that its outer surface is exactly flat) the face 48 of thefree end of the stem centered by the armature stem guide 44 ispositioned approximate to the face of the exhaust nozzle 46.

I If the face of the armature stem touches the nozzle, it seals offpassage of fluid from' the probe cavity to the fluid exhaust port 19. Ifthe armature stem is lifted off the nozzle, a passage for fluid isprovided. The effective size of the passage and hence its resistance tofluid flow depend upon the extent of separation between the-armaturestem end and the nozzle. The face of the armature stem then constitutesa flapper 48 working against the nozzle 46. The operation of an orificeand a nozzleflapper couple is well known in the art.

Operation of the probe will now be discussed. First of all, assume thefollowing condition: 1) Thefluid supply pressure is substantiallygreater than the pressure to be measured which acts upon the outsidesurface of the diaphragm; 2) the fluid exhaust pressure is substantiallyless than the pressure to be measured; 3) the biasing spring 49extending between the diaphragm armature 40 and the armature stem guide44 is adjusted to exert no force on the diaphragm with the latter in itsneutral position.

In situation I, with the probe in a condition of equilibrium, pressureinside the probe substantially equals that outside, so that forcesacting on the diaphragm, which is in its neutral position, exactlybalance. At this neutral position of the diaphragm the position of theflapper 48 1 relative to the nozzle 46 is such as to produce thepressure existing inside the probe.

In situation II suppose the pressure acting on the outside surface ofthe diaphragm increases; since the j forces no longer exactly balance,the flapper tends to move closer to the nozzle. As this happens, itsrestrictive effect on fluid flow through the system and out through thenozzle increases and pressure inside the probe correspondinglyincreases. A new condition of equilibrium will be reached when thepressure inside the probe again substantially equals that outside. Itwill be noted that the flapper must be slightly closer to the nozzle toproduce a new higher pressure inside the probe. However, due to theextreme sensitivity of pressure within the probe to flapper position,this sustained position change may be only a small fraction of athousandth of an inch.

In situation III the pressure outside the diaphragm 18 drops and theflapper 48 moves away from nozzle 46 very slightly to a new positionwith respect to the nozzle. This produces a new lower pressure insidethe probe substantially equal to the new lower pressure outside.

In this manner pressure changes within the probe and in any dead endedsystem connected to it, follow pressure changes acting on the outsidesurface of the diaphragm. v

The above description describes the action when the biasing springexerts no force on the diaphragm. The situation will now be describedwhen the biasing spring is adjusted to pull inwardly on the diaphragm,18. At a condition of equality of pressures acting on the inside andoutside of the diaphragm, the system then would no longer be an exactbalance since the spring force is unopposed. An additional increment offlapper position movement towards the nozzle takes place until anadditional increment of pressure inside the probe is produced whichacting upon the effective area of the diaphragm creates an additionalincrement of force to balance the spring force. Now a new balance existsand the pressure inside the probe equals that outside plus an incrementdue to spring pull. Since the spring pull is substantially constant, itfollows that the pressure inside the probe is always higher than thatoutside by a constant amount. However, changes in pressure inside arealways substantially equal to changes in outside pressure.

In the preceding description of operation it was pointed out that inorder for the probe to follow an increased pressure applied to theoutside of the diaphragm it was necessary that the flapper assume a newposition slightly closer to the nozzle. This increment of positionchange, no matter how small, associated with the rate of the biasingspring, spring rate of the dia-' phragm due to its elasticity, theeffective area change of the diaphragm, and the changes in reactionforce due to fluid flow around the repositioned flapper, result inpressure changes within the probe substantially but not exactly equal topressure changes outside. For highly precise reproduction of pressurechanges this error is not tolerable especially in cases where themeasured pressure varies widely.

For given supply and exhaust pressures, given orifice and nozzle sizes,and a given position of the flapper with respect to the nozzle withinits throttling range, the pressure within the probe will be at someintermediate value between supply and exhaust pressures. If both supplyand exhaust pressures are changed by exactly the same given amount, theintermediate pressure will also change by exactly the given amountwithout requiring any change whatsoever in flapper position relative tothe nozzle since pressure drop across the system' and therefore fluidflow through the system and pressure drop across the orifice, will allremain constant. By changing supply and exhaust pressures in one-to-onerelationship to probe output pressure changes, the latter can be made tofollow pressure changes acting on the outside surface of the diaphragmwithout any sustained position changes of the flapper and the asso-.

ciated errors.

As shown in FIG. 1 the output pressure P of the probe is used as a pilotpressure for a pair of fluid control regulators 60 and 62. Regulator 60maintains fluid pressure at the supply port of the probe and regulator62 maintains fluid pressure at the exhaust port 19. The supply regulator60 is spring biased by a value C to maintain its downstream pressure ata value always a selected increment above its pilot pressure. Similarlythe exhaust regulator 62 is biased by a value C to maintain its upstreampressure at a value always a selected increment below its pilotpressure. Both regulators control fluid flow to accomplish thisregulation. Construction and operation of regulators, the function ofwhich are described above, are well known in the art and arecommercially available. Therefore description of their operation willnot be detailed here.

The pressure sensing probe system shown in duplicate in FIG. 1 producesan output pressure, changes in which follow in a one-to-one relationshipto changes in pressure applied to the outside surface of diaphragm 18.In equilibrium it is an essentially perfect null balance device, henceit has no error proportional to magnitude of the pressure measured.Resolution is infinite. It may be biased to produce an output pressurehigher or lower than the measuredpressure by any selected constantamount. .The probes speed of response is a function of orifice andnozzle size which determine maximum rates of rise and fall respectivelyof pressure within the probe and any connected dead-end system.

An alternative pressure sensing probe construction is illustrated inFIG. 4. In this embodiment the supply fluid enters the probe throughnozzle 146 and exhausts through orifice a. Analysis similar to theforegoing shows that the operation of this type probe is equivalent tothat of the probe shown in FIG. 3 where the supply fluid enters throughthe orifice 15a and exhausts through the nozzle 46. It will be noted inFIG. 4 that nozzle flapper operation is the reverse of that of FIG. 3,in that an inwardly moving diaphragm moves the flapper 148 away from thenozzle, not toward it. This embodiment additionally has a centeringdiaphragm for mounting the stem 141 and a bias adjusting stem 154attached to one end of biasing spring 149. A protective cap 156 coversthe endof the bias adjusting stem. The remaining elements of thealternative pressure sensingprobe are essentially the same as that ofthe probe described in FIG. 3 with the numbers of the elements in FIG. 4all being preceded by a one hundred numeral.

While the foregoing descriptions have stressed the use of the pressuresensing probe as applied to a Venturi tube carrying contaminated fluid,it is evident that the probe may be used in any application wherepressure must be sensed, particularly where highly precise 'results arerequired. It may, for instance, be used to gage levels in Parshallflumes, weirs, Kennison nozzles, etc., or in tanks, reservoirs,standpipes, wells, etc. It may be used for pressure, level, flow, etc.measurement of dangerous as well as solids-bearing fluids. Many otherapplications are possible and practical.

What is claimed is: 1. A pressure sensing probe comprising a housing, anelastic diaphragm located in one wall of said housing, an interiorcavity formed in said housing, at least a portion of the inner face ofsaid diaphragm being in communication with said interior cavity, a fluidsupply port formed in a wall of said housing with saidfluid supply portbeing in communication with said interior cavity, a fluid outputpressure port formed in a wall of; said housing with said fluid outputpressure port being in communication with said interior cavity, v afluid exhaust port formed in a wall of said housing with said fluidexhaust port being in communication with said interior cavity, meansincluding ,a nozzle between the supply and exhaust ports and a flapperto vary the fluid flow through said cavity, said flapper coupled to saiddiaphragm, f a fluid supply regulator connected to said fluid supplyport and a fluid exhaust pressure regulator connected to said fluidexhaust port and means connecting said fluid output pressure port toeach of said regulators, said fluid supply regulator maintaining fluidsupply pressure to said fluid supply port a fixed increment above thepressure at said output pressure port and said fluid exhaust regulatormaintaining pressure from said fluid exhaust port a fixed incrementbelow pressure at said output pressure port, a flow restricting orificebetween said fluid supply regulator and said interior cavity.

2. A pressure sensing probe as recited in claim 1 wherein the outer faceof said diaphragm is substantially flush with the outer wall surface ofsaid housing.

3. A pressure sensing probe as recited in claim 1 further comprisingmeans for biasing said diaphragm comprising an armature attached to theinner face of said diaphragm and a biasing spring having its one endattached to said armature and its other end attached to spring anchoringmeans on said housing.

4. A pressure sensing probe as recited in claim 3 wherein said armaturehas a stern that passes downwardly within said biasing spring andthrough a guide aperture formed in said spring anchoring means.

5. A pressure sensing probe as recited in claim 3 wherein the lower endof said armature stem forms a flapper which when it rises and fallsvaries the flow of fluid through said nozzle.

6. A pressure sensing probe as recited in claim 3 including a centeringdiaphragm in said cavity wherein the lower end of the armature passesthrough said centering diaphragm to which it is secured.

7. A pressure sensing probe as recited in claim 9 wherein a biasingspring is attached to the lower end of the armature and to the housingtogether with means for adjusting the bias of said spring.

8. A pair of the pressure sensing probes such as the probe recited inclaim 1 in combination with a Venturi tube, said first pressure sensingprobe having the outer face of its flexible diaphragm inserted in a tapin the inlet area of the Venturi tube and said second pressure sensingprobe having the outer face of its flexible diaphragm inserted in a tapin the throat area of the Venturi tube, both of the outer faces of saidflexible diaphragms, being substantially flush with the inner wallsurface of said Venturi tube.

. UNITED STATES PATEN'Ij OFFICE .CERTIFICATE OF CORRECTION Patent NO.Dated I 2,

Inventor( 1!) Wilbur H. F razel It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

In column 8, line 8, cancel the numeral "9" and insert in lieu thereofthe numeral" 6 Signed and sealed this 5th day of November 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting'officer Commissioner ofatents USCOMM-DC 50376-P59 i U.S. GOVERNMENT PRINTING OFFICE I",0-358-35,

FORM PO-1050 (10-69)

1. A pressure sensing probe comprising a housing, an elastic diaphragmlocated in one wall of said housing, an interior cavity formed in saidhousing, at least a portion of the inner face of said diaphragm being incommunication with said interior cavity, a fluid supply port formed in awall of said housing with said fluid supply port being in communicationwith said interior cavity, a fluid output pressure port formed in a wallof said housing with said fluid output pressure port being incommunication with said interior cavity, a fluid exhaust port formed ina wall of said housing with said fluid exhaust port being incommunication with said interior cavity, means including a nozzlebetween the supply and exhaust ports and a flapper to vary the fluidflow through said cavity, said flapper coupled to said diaphragm, afluid supply regulator connected to said fluid supply port and a fluidexhaust pressure regulator connected to said fluid exhaust port andmeans connecting said fluid output pressure port to each of saidregulators, said fluid supply regulator maintaining fluid supplypressure to said fluid supply port a fixed increment above the pressureat said output pressure port and said fluid exhaust regulatormaintaining pressure from said fluid exhaust port a fixed incrementbelow pressure at said output pressure port, a flow restricting orificebetween said fluid supply regulator and said interior cavity.
 2. Apressure sensing probe as recited in claim 1 wherein the outer face ofsaid diaphragm is substantially flush with the outer wall surface ofsaid housing.
 3. A pressure sensing probe as recited in claim 1 furthercomprising means for biasing said diaphragm comprising an armatureattached to the inner face of said diaphragm and a biasing spring havingits one end attached to said armature and its other end attached tospring anchoring means on said housing.
 4. A pressure sensing probe asrecited in claim 3 wherein said armature has a stem that passesdownwardly within said biasing spring and through a guide apertureformed in said spring anchoring means.
 5. A pressure sensing probe asrecited in claim 3 wherein the lower end of said armature stem forms aflapper which when it rises and falls varies the flow of fluid throughsaid nozzle.
 6. A pressure sensing probe as recited in claim 3 includinga centering diaphragm in said cavity wherein the lower end of thearmature passes through said centering diaphragm to which it is secured.7. A pressure sensing probe as recited in claim 9 wherein a biasingspring is attached to the lower end of the armature and to the housingtogether with means for adjusting the bias of said spring.
 8. A pair ofthe pressure sensing probes such as the probe recited in claim 1 incombination with a Venturi tube, said first pressure sensing probehaving the outer face of its flexible diaphragm inserted in a tap in theinlet area of the Venturi tube and said second pressure sensing probehaving the outer face of its flexible diaphragm inserted in a tap in thethroat area of the Venturi tube, both of the outer faces of saidflexible diaphragms, being substantially flush with the inner wallsurface of said Venturi tube.